The present invention relates, in general, to sensing structures and, more particularly, to sensing structures comprising a movable mass, such as found in an accelerometer, and a self-test capability for verifying proper operation of the movable mass.
Sensors are one type of electronic device using a movable mass. In particular, accelerometers use a movable mass to detect acceleration. Typically, a differential capacitor is connected to the movable mass, and motion of the mass is detected by a sensing circuit coupled to the differential capacitor.
Accelerometers used in air-bag deployment systems require a self-test capability under conditions closely resembling an actual crash event. Typical surface-micromachined silicon accelerometers accomplish this self-test function by creating an electrostatic attraction of the movable or seismic mass by biasing an electrode in the sensor. However, unless the forces of electrostatic attraction are applied symmetrically about the center of mass, the resulting imbalance can cause rotational motion of the mass. For a linear acceleration sensor, this rotational or angular motion can produce errors, and in extreme cases permanently disable the sensor.
Accordingly, there is a need for an accelerometer with a self-test capability in which the movable mass structure can be displaced in purely linear fashion along the sensitive axis.